There was a guy advertising this kind of work on the web. I do not recall the name of his outfit. Found it- Kurts Kustom Firearms. I do not know if he is still in business though. He made rails and folding charging handles.
I used to have one of these (regret selling it but that is what happens in divorces). Most have enough threads to install an M-16 flash hider or muzzle brake. Take off the two slotted nuts and the stupid over barrel sleeve and screw on the FH. The end cap of the receiver can be omitted or secured with a small set screw like I had.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball